1. Field of the Invention
The present invention relates to a process migration method, computer and system for copying, to another computer node, the process currently executed on a certain computer node, and a computer having such a copying function.
2. Description of the Related Art
A computer system formed of a plurality of computer nodes employs a process migration technique for copying, to another computer node, the process currently executed on a certain computer node, in order to reduce the load on a certain computer.
Specifically, the process migration technique is a technique for copying, to another computer node, the state of the process (process context) currently executed on a certain computer node, thereby enabling another computer to continue the execution of the process using the copied process context. The process context specifically means data (including codes) in a memory space used in the process, register values indicative of the state of a central processing unit (CPU), and information in a file opened in the process, etc.
In a computer system including a plurality of computer nodes, if a load is concentrated on a certain computer node, and other computer nodes have a spare processing capability, the process migration technique enables the process executed on the overloaded computer node to be copied to other computer nodes, thereby equalizing the load on the computer nodes. In general, if the load on a computer system is equalized, the response time and throughput of the computer system can be enhanced.
Since recent computer nodes incorporate a large-capacity memory to enhance their processing capability, there is a growing tendency for a large memory capacity to be needed in a process. Accordingly, the amount of data of a process context is becoming very large. The period required for data communication between computer nodes increases with the amount of communication data. Therefore, the period required for a process context to be copied between computer nodes is increasing.
In the conventional process migration technique, to prevent a process context from being updated during process migration, the execution of the process to be copied is once stopped, as is disclosed in, for example, Jan. Pat. Appln. KOKAI Publication No. 2000-222376. Therefore, a user utilizing the computer system may think the computer system is hung.
Further, “Preemptable and remote execution facilities for the V-system” (M. Theimer, K. Lantz and D. Cheriton in 10th ACM Symposium on Operating Systems Principles, pages 2-12, 1985) discloses a technique called “PreCopy”. In this technique, all contents of a process context in a memory space used in a process are copied to a destination computer node during the execution of the process on an originator computer node. After the copying operation, the process is continued and then finished on the originator computer node. When this process is finished, the contents of the process context, which have been changed on the originator computer node after they are copied to the destination computer node, are also copied to the destination computer node, together with data indicative of the state of the process.
However, if a lot of time is required for copying the contents in the memory space, as stated above, a large amount of contents are changed by the execution of the process during copying, therefore a lot of time is required to copy data indicative of the changed contents to the destination computer node. As a result, a user may think that the computer system is hung for a long time.
As described above, in the conventional process migration technique, the execution of a process is temporarily stopped. Since, however, there is a tendency for the period required for copying a process context between computer nodes to increase, a user utilizing the computer system may think that the computer system is hung for a long time.